mcrypt: use /dev/urandom instead of /dev/random

/dev/random use keyboard and mouse inputs to generate random numbers.
But on a server, these inputs don't exist, so /dev/random can take more
time to generate the random numbers (too much time).

/dev/urandom doesn't have this problem, normally.
Stronger security for encryption/decryption

Use MCRYPT_MODE_CBC instead of MCRYPT_MODE_ECB. This implies to store
the 'iv' (a "seed" or a "salt").